Fizyka - program rozszerzony I
- Wprowadzenie do obwodów i prawo Ohma
- Rezystywność i przewodność właściwa
- Current, resistance, and resistivity review
- Electric potential difference and Ohm's law review
- Obliczanie oporu, napięcia i prądu za pomocą prawa Ohma
Review the key terms, equations, and skills related to Ohm's law, including how electric potential difference, current, and resistance are related.
|Battery||Device that transforms chemical energy into electrical energy. An ideal battery has no internal resistance.|
|Electric potential difference (||Energy change per unit charge between two points. Also called voltage or electric potential. Has SI units of Volts |
|Electromotive force (EMF, ||EMF is the potential difference produced by a source such as an ideal battery. Has SI units of |
|Equation||Symbols||Meaning in words|
|Current is directly proportional to electric potential difference and inversely proportional to resistance.|
Ohm’s law states that for some devices there is a relationship between electric potential difference, current, and resistance.
The equation is:
is current, is electric potential difference, and is resistance.
How are electric potential difference and current related?
For a given resistance
, increasing the electric potential difference increases the current and vice versa.
How are current and resistance related?
For a given electric potential difference
, if the resistance increases, then the current decreases and vice versa.
How are resistance and electric potential difference related?
For a given current
, if the electric potential difference increases, then the resistance also increases and vice versa.
Analyzing electric potential difference across a resistor using Ohm’s law
If the current encounters resistance, the electric potential difference decreases according to Ohm’s law. We sometimes call this a voltage drop.
Analyzing electric potential difference and current across a battery
A common source of electric potential is a battery, which is represented in diagrams by the symbol below (Figure 2). The short side is the negative end, with a lower electric potential, and the long side is the positive end, with a higher electric potential.
Electrons flow from the negative terminal to the positive terminal. Conventional current
travels from the positive terminal (higher electric potential), through the circuit, and finally to the negative end (lower electric potential).
Current flow and electric potential difference can be better understood by using the analogy of a boulder rolling down a hill. At the top of the hill, the boulder has a lot of gravitational potential energy. Similarly, an electron has a lot of stored energy in the form of electric potential energy when it is at the negative terminal of a battery. The boulder will naturally fall toward the ground where potential energy is lower. The electron at the negative terminal of a battery will naturally flow toward the positive terminal, where the electric potential is lower.
As the boulder falls downward, the stored energy is converted to kinetic energy. As the electron flows across electrical components, the stored energy is converted into various forms of energy such as heat and light.
Często spotykane błędy i nieporozumienia
Sometimes people think all devices follow Ohm’s law. However, a device is only ohmic when the current is directly proportional to the electric potential difference, and inversely proportional to the resistance. If we plotted an electric potential vs. current graph for an ohmic device, the relationship would be linear (see Figure 3).
Some devices such as light bulbs are non-ohmic. This means that their electric potential difference-current graphs are non-linear, as in Figure 3. For non-ohmic devices, we can’t use
to solve for an unknown.